"Alzheimer's disease has a burden that exceeds many of these other illnesses," especially because of how long people live with it and need care, he said.
For the new study, researchers started with about 11,000 people in a long-running government health survey of a nationally representative sample of the population. They gave 856 of these people extensive tests to determine how many had dementia, and projected that to the larger group to determine a prevalence rate - nearly 15 percent of people over age 70.
Using Medicare and other records, they tallied the cost of purchased care - nursing homes, medicines, other treatments - including out-of-pocket expenses for dementia in 2010. Next, they subtracted spending for other health conditions such as high blood pressure, diabetes or depression so they could isolate the true cost of dementia alone.
"This is an important difference" from other studies that could not determine how much health care cost was attributable just to dementia, said Dr. Kenneth Langa, a University of Michigan researcher who helped lead the work.
Even with that adjustment, dementia topped heart disease and cancer in cost, according to data on spending for those conditions from the federal Agency for Healthcare Research and Quality.
Finally, researchers factored in unpaid care using two different ways to estimate its value - foregone wages for caregivers and what the care would have cost if bought from a provider such as a home health aide. That gave a total annual cost of $41,000 to $56,000 per year for each dementia case, depending on which valuation method was used.
"They did a very careful job," and the new estimate that dementia affects about 4.1 million Americans seems the most solidly based than any before, Hodes said. The government doesn't have an official estimate but more recently has been saying "up to 5 million" cases, he said.
The most worrisome part of the report is the trend it portends, with an aging population and fewer younger people "able to take on the informal caregiving role," Hodes said. "The best hope to change this apparent future is to find a way to intervene" and prevent Alzheimer's or change its course once it develops, he said.